Wednesday, August 24, 2011

Play My Code is a place where you can create your own online games easily using the site's Quby language. The games are compiled into native JavaScript, compatible with all modern HTML5-compliant browsers. Once you've written a game, you can embed it on your website or blog, like I have done below. It's like YouTube, but for games!

Here's Chain Reaction! The aim of the game, is to start a chain reaction (by clicking on the screen) and explode as many atoms as possible. See how many levels you can complete!

Sunday, August 21, 2011

The ThreadLocalRandom class in JDK 7, allows you to generate random numbers from multiple threads. It is more efficient than using shared Random objects and will result in better performance as there is less overhead and contention.

In addition, this class also provides "bounded" generation methods.

For example, the statement below generates a random number between 1 (inclusive) and 100 (exclusive).

int random = ThreadLocalRandom.current().nextInt(1,100);

Wait, there's a BUG!
While trying out this class, I noticed that the SAME random numbers were being produced across all my threads. I then discovered this bug which reported the same issue I was having. It appears that the seed is never initialised so the same random numbers are produced every time. I wouldn't recommend using this class until the bug is fixed. The following code illustrates the issue:

Saturday, August 20, 2011

A really useful feature of the Unix less pager is LESSOPEN which is the "input preprocessor" for less. This is a script, defined in the LESSOPEN environment variable, which is invoked before the file is opened. It gives you the chance to modify the way the contents of the file are displayed. Why would you want to do this? The most common reason is to uncompress files before you view them, allowing you to less GZ files. But it also allows you to list the contents of zip files and other archives. I like to use it to format XML files and to view Java class files by invoking jad.

Sunday, August 14, 2011

I wrote about my Eclipse code templates a few years ago and since then I've made a quite a few changes to them. I've added a few new templates to help with JUnit tests and xml parsing. I've also updated my existing file IO templates to use Java 7 features.

Templates are simply "magic words" or shortcuts to standard blocks of code or text. They are very handy because once you have them setup you don't have to waste time writing boilerplate code any more! An example of a pre-defined template in Eclipse is sysout which expands to System.out.println();. All you have to do is type sysout followed by Ctrl+Space to insert the statement into your Java source file.

To see what templates are defined in Eclipse:

Open your Preferences dialog by going to Windows > Preferences

On the navigation tree on the left, go to Java > Editor > Templates

You will see a list of pre-defined templates

You can add new ones by pressing the "New..." button

My templates are shown below. They can also be downloaded from my GitHub repository and then imported into Eclipse.

File IO Templates:
The following templates are useful for reading or writing files. They use Java 7 features such as try-with-resources to automatically close files. They also use methods from NIO2.0 to obtain a buffered reader and read the file.

The java.library.path system property instructs the JVM where to search for native libraries. You have to specify it as a JVM argument using -Djava.library.path=/path/to/lib and then when you try to load a library using System.loadLibrary("foo"), the JVM will search the library path for the specified library. If it cannot be found you will get an exception which looks like:

Exception in thread "main" java.lang.UnsatisfiedLinkError: no foo in java.library.path
at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1734)
at java.lang.Runtime.loadLibrary0(Runtime.java:823)
at java.lang.System.loadLibrary(System.java:1028)

The java.library.path is read only once when the JVM starts up. If you change this property using System.setProperty, it won't make any difference.

Here is the code from ClassLoader.loadLibrary which shows how the path is initialised:

Option 2: Add path to usr_paths
Instead of having to re-evaluate the entire java.library.path and sun.boot.library.path as in Option 1, you can instead append your path to the usr_paths array. This is shown in the following code:

Saturday, August 13, 2011

I've added all my dotfiles (including my entire bash profile and vimrc) to my GitHub dotfiles repository. Whenever I make any changes, I will commit them to the repository.

In order to download the latest version, go to my Downloads page. Alternatively, if you have git installed, use the following command, to clone my repository:

git clone git://github.com/sharfah/dotfiles.git

This will download them to a directory called dotfiles. You can then copy the files recursively (cp -r) to your home directory (don't forget to backup your original files first!). Alternatively, use symlinks.

Sunday, August 07, 2011

The Watch Service API in JDK7 allows you to watch a directory for changes to files and receive notification events when a file is added, deleted or modified. You no longer need to poll the file system for changes which is inefficient and doesn't scale well.

The code below shows how you would use the Watch Service API. First, you have to create a WatchService for the file system and then register the directory you want to monitor with it. You have to specify which events (create, modify or delete) you are interested in receiving. Then start an infinite loop to wait for events. When an event occurs, a WatchKey is placed into the watch service's queue and you have to call take to retrieve it. You can then query the key for events and print them out.

The Zip File System Provider in JDK7 allows you to treat a zip or jar file as a file system, which means that you can perform operations, such as moving, copying, deleting, renaming etc, just as you would with ordinary files. In previous versions of Java, you would have to use ZipEntry objects and read/write using ZipInputStreams and ZipOutputStreams which was quite messy and verbose. The zip file system makes working with zip files much easier!

This post shows you how to create a zip file and extract/list its contents, all using a zip file system.

Constructing a zip file system:
In order to work with a zip file, you have to construct a "zip file system" first. The method below shows how this is done. You need to pass in a properties map with create=true if you want the file system to create the zip file if it doesn't exist.

Once you have a zip file system, you can invoke methods of the java.nio.file.FileSystem, java.nio.file.Path and java.nio.file.Files classes to manipulate the zip file.

Unzipping a Zip File:
In order to extract a zip file, you can walk the zip file tree from the root and copy files to the destination directory. Since you are dealing with a zip file system, extracting a directory is exactly the same as copying a directory recursively to another directory. The code below demonstrates this. (Note the use of the try-with-resources statement to close the zip file system automatically when done.)

Saturday, August 06, 2011

I don't like the fact that Eclipse defaults to "Java Search" whenever you open the Search Dialog (Ctrl+H), because I almost ALWAYS need "File Search". I then need to press Ctrl+Page Up to switch to the "File Search" tab. The good news is that there is a way to make Eclipse default to "File Search" by changing your key bindings. This is how:

Go to Window > Preferences and navigate to General > Keys

Type Open Search Dialog in the filter text box to search for the command. You should see it bound to Ctrl+H. Click on the command and press Unbind Command

Now type File Search and click on the command. In the Binding text field enter Ctrl+H

Press Apply

Now, whenever you hit Ctrl+H, the Search Dialog will show the File Search tab by default!

Another thing...
You can clean up the Search Dialog by removing those tabs that you don't need. Do this by opening the Search Dialog (Ctrl+H) and then clicking on Customize.... Deselect the ones you don't need e.g. Task Search, JavaScript Search, Plug-in Search, Spring Pointcut Matches etc.

inputrc is the name of the readline startup file. You can set key bindings and certain variables in this file. One of my favourite key bindings is Alt+L to ls -ltrF. I also have bindings which allow you to go back and forth across words using the Ctrl+Left/Right Arrow keys.

To take a look at all your current key bindings execute the command bind -P or bind -p. Check out the man pages for more information.

set bell-style none
set completion-ignore-case On
set echo-control-characters Off
set enable-keypad On
set mark-symlinked-directories On
set show-all-if-ambiguous On
set show-all-if-unmodified On
set skip-completed-text On
set visible-stats On
"\M-l": "ls -ltrF\r"
"\M-h": "dirs -v\r"
# If you type any text and press Up/Down,
# you can search your history for commands starting
# with that text
"\e[B": history-search-forward
"\e[A": history-search-backward
# Use Ctrl or Alt Arrow keys to move along words
"\C-[OD" backward-word
"\C-[OC" forward-word
"\e\e[C": forward-word
"\e\e[D": backward-word
"\M-r": forward-search-history

If you have any useful bindings, please share them in the comments section below.

Tuesday, August 02, 2011

The Java 7 NIO library allows you to walk a file tree and visit each file in the tree. You do this by implementing a FileVisitor and then calling Files.walkFileTree using the visitor. The visitor has four methods:

visitFile: Invoked for a file in a directory.

visitFileFailed: Invoked for a file that could not be visited.

preVisitDirectory: Invoked for a directory before entries in the directory are visited.

postVisitDirectory: Invoked for a directory after entries in the directory, and all of their descendants, have been visited.

Recursively Delete all Files in a Directory:
The following code shows how you can recursively delete a directory by walking the file tree. It does not follow symbolic links. I have overridden the visitFile and postVisitDirectory methods in SimpleFileVisitor so that when a file is visited, it is deleted and after all the files in the directory have been visited, the directory is deleted.

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Hi, I'm Fahd, a software developer at an investment bank in London. I am passionate about technology and work mainly with open source software, specialising in Java applications and Unix-based operating systems.

This blog is a place for me to share useful code snippets to solve problems that I have come across, and to write about ideas and experiences as a programmer.

All code on this blog has been written by me, unless stated otherwise, and you are free to use, share and adapt it for any purpose, under the terms of the GNU General Public License.

I love hearing back from my readers, so please feel free to leave comments! Thanks for reading and happy programming :-)